They really do add on to the actual damage you inflict. This means it is unaffected by modifiers such as monster defense. I tested this out on an Orc with a sub level 10 archer, and for every extra STR I add I did 1 more damage. With such a huge discrepancy between the Orc's defense and my pathetic display damage, I can only attribute the damage increase solely to the +STR.
I just thought this might interest people who favour +SPR over +STR. For maximum actual damage: the amount of extra damage from critical hits, spread out over a large number of hits, cannot match the +damage from +STR until your average actual damage exceed 1000 by a margin. While this might be done versus things half your level, in normal circumstances you can only dream about 4 digit average damage.
At level 50, 55 points into STR would give a whooping 66 extra damage per hit, which is independent of the thing you're attacking. I'd think that means a lot to clerics hammering things by themselves.
I suspect the +Mdamage from +INT works the same too.
It would seem full INT mage > Fission